The present invention relates generally to transfer switches and, more particularly, to a transfer switch which detects the presence and provides an indication of ground currents.
Electrical panels, breaker boxes, or load centers frequently include a main contactor, switch, or breaker, which electrically isolates a series of individual circuit or load breakers from a utility power input. In a typical utility power input installation, the main contactor selectively connects or disconnects a first hot lead, L1, and a second hot lead, L2, with a respective bus bar, to which the individual circuit or load breakers are connected. A neutral lead, N, is connected to a neutral bar from which neutral connections are distributed to each of the loads. In addition, a ground connection is established in the load center, for example, by a ground bar connected to a conductive rod inserted into the ground or to a metal wire or pipe exiting the site into the ground. The ground bar provides connection points for ground conductors to be distributed to the loads as required. According to wiring standards, the neutral lead, N, from the utility power is connected to the ground connection at a single location within the electrical distribution system. Thus, in a standard installation, a bonding wire may be connected between the neutral bar and the ground bar in the load center.
The load center may also be configured to receive a secondary input power source, such as from a generator, to provide electrical power to certain of the individual loads or circuits in the event of a utility power failure. During interruption of utility power, the generator supplies power to the load center, which the load center distributes to selected circuits of the building. Depending on factors such as the size of the generator, the number of electrical loads, and whether a load is considered critical (i.e., must remain on during a utility power outage), such as a furnace, sump pump, etc., the secondary power source may power all of the electrical loads or only a portion of the loads. Similar to the utility power input, the generator includes a first hot lead, L1, a second hot lead, L2, and a neutral lead, N. The generator may also include a ground connection, such as a receptacle grounding terminal. The generator maybe configured either with a bond between the neutral lead, N, and the ground connection (i.e., a bonded-neutral generator) or without a bond between the neutral lead, N, and the ground connection (i.e., a floating-neutral generator).
In a non-separately derived system, the neutral lead, N, from the generator is solidly connected to the neutral connection from the utility supply, and the neutral lead, N, of the generator must neither be directly connected to an earth ground nor to the ground connection of the generator (i.e., a floating-neutral generator). As previously indicated, a connection between the neutral and ground leads should be established at a single location in the electrical distribution system. Because the neutral connection in the generator is floating, the connection between the neutral and ground leads is established in the service entrance panel. In the same manner as a distribution system having no generator, this connection may be established via a bonding wire connected between the neutral bar and the ground bar in the load center. The neutral lead from the generator is bonded with the neutral lead from the utility, for example, at the neutral bar. A transfer switch is provided to selectively connect the hot leads, L1 and L2, from either the utility or the generator to the electrical loads.
A generator that has its neutral lead, N, bonded to the ground connection of the generator (i.e., a bonded-neutral generator) must be installed as a separately derived system. The neutral conductor from the loads is switched between the neutral lead, N, of each of the power sources to maintain a single bonding point in the system. The neutral leads, N, of the utility power source and the secondary power source, such as the generator, are switched by the transfer switch in addition to the hot leads, L1 and L2. Further, the connection between neutral and ground for the utility system must occur prior to the switched neutral connection such that it is removed when the secondary power source is connected and the equipment grounds for the loads are separated from the neutral. Typically the neutral bar and the ground bar are not connected directly in a service entrance panel but rather they are combined into one dual-function terminal bar. This separation is needed in a service entrance transfer panel to keep the neutral and ground conductors separate when switched to the secondary power source since the connection between neutral and ground takes place further upstream toward the power source and cannot occur again in the panel.
A first potential problem arises when a bonded-neutral generator is connected to a service panel having a transfer switch that is only configured to switch the hot leads, L1 and L2. In such a configuration, multiple connection points can be established between the neutral leads and the ground leads, creating a second conduction path parallel to the neutral conduction path. As a result, a portion of the operating current for the electrical loads that is supposed to be conducted via the neutral lead may be present on the ground lead, resulting in an undesirable current on the ground lead.
A second potential problem arises when a bonded-neutral generator is connected to a service entrance panel having a transfer switch that is configured to switch the neutral lead, N, in addition to the hot leads, L1 and L2, in that the neutral switching portion of the transfer switch may fail. In the event the neutral portion of the transfer switch fails, the neutral lead, N, is no longer switched, and the neutral lead, N, of one of the power sources is always connected to the loads. When the power is supplied to the loads via the hot leads, L1 and L2, from the other power source, the power must return via the ground lead because the neutral lead, N, is no longer available. Again, an undesirable current is present on the ground lead.